Effect of photoinhibition during bottle incubations on the measurement of seasonal primary production in a shallow coastal water

Abstract

Primary productivity in Vineyard Sound, Massachusetts (USA), a shallow, well-mixed coastal water, was determined at monthly intervals during May-Oct., 1981. Over the course of each study day fresh samples were obtained at 3 h intervals and then incubated until sunset with time-course measurements of H14CO3- uptake made at frequent intervals. Two seasonal patterns of 14C uptake were apparent. First, rapid initial uptake of 14C over each incubation during the spring and fall was folllowed by reduced uptake during periods of strong sunlight. The duration over which rapid uptake was sustained was a function of the prevailing light intensity at the start of the incubation. Under these conditions estimates of near surface primary productivity based on summing total production over the first 3 h of each back to back incubation were higher by 50-130% than estimates based on single end-point measurements from full-day incubations. During summer, uptake of 14C was linear throughout each incubation when both total daily irradiation and temperature were maximal so that the 2 techniques for estimating productivity led to comparable results. The reasons for these seasonal differences in the pattern of 14C uptake are unclear, but may be related to changes in temperature or phytoplankton sepcies, or both. These results for a shallow, well-mixed water are in general accord with the conclusion of Harris (1980) that when vertical mixing is sufficient to prevent exposure of phytoplankton to photoinhibiting light intensities at the surface, photoinhibition could be far more pronounced during a bottle incubation than is occurring in the water column.